The human body contains various tissues that continually undergo a process of self-renewal, whereby older cells in the tissue die and are replaced by new cells. In order to maintain a constant number of cells within a particular tissue, it is important that the number of newly produced cells equals the number of cells that die. This homeostasis is maintained by committing differentiated cells to a deliberate and genetically controlled cellular process known as programmed cell death or apoptosis. Apoptotic cells undergo characteristic morphological changes including cell shrinkage, loss of mitochondrial function, and both nuclear condensation and fragmentation. These cellular alterations provide structures suitable for recognition and clearence by proximal phagocytosing cells. Importantly, apoptosis occurs without inducing an inflammatory response and without damage to surrounding cells.
Apoptosis can be induced by a number of unrelated stimuli. However, recent evidence suggests that regardless of the initiating stimulus, apoptosis is signalled through a common pathway. Numerous genes associated with this pathway have been identified, but the way in which their products interact to execute the apoptotic program is still poorly understood.
Defects in the apoptotic pathway can contribute to the onset or progression of various pathological conditions. In humans, the failure of cells to undergo appropriate apoptosis can lead to cancer, autoimmune diseases and viral infection. Conversely, accelerated rates of apoptosis can lead to e.g., neurodegenerative disorders and osteoporosis. Thus, controlling inappropriate cell death or cell survival is important for the treatment of a variety of human diseases.
Amongst the few proteins known to inhibit cell death are certain members of the Bcl-2 family of proteins (Reed, Nature 387:773-776). Recently, another family of anti-apoptotic proteins, IAP (inhibitor of apoptosis), was identified (Clem and Duckett, Trends in Cell Biology 7:337-339).
The first IAP gene was identified in baculovirus and since then cellular homologues of IAP have been identified in Drosophila, chickens and humans (Hay et al., Cell, 83, 1253-1262, 1995; Duckett et al., EMBO J., 15:2685-2689, 1996; Liston et al., Nature 379:349-353, 1996). The IAP proteins are highly conserved through evolution and characteristically contain two types of sequence motifs/domains (Reed, supra). The C-terminus of an IAP typically contains a RING finger motif. This motif is a type of zinc finger motif, and is thought to be involved in protein-protein interactions. However the exact function of the RING finger motif remains elusive (Saurin et al., Trends Biochem Sci 21:208-214, 1996).
The other common sequence motif common to many of the IAPs is a BIR (baculovirus IAP repeat). BIRs, which are situated at the N-terminus of the IAP, normally comprise 2-3 imperfect repeats of approximately 65 amino acid residues each and contain a number of absolutely conserved residues, including CysX.sub.2 Cys and CysX.sub.6 His motifs (where X is any amino acid). Recent evidence suggests that the BIRS mediate anti-death activity through their involvement in protein-protein interactions (Ambrosini et al., Nature Med 3:917-921, 1997). To date, all IAPs have been found to contain at least one BIR motif. Furthermore, all but two members of the IAP protein family, NAIP (neuronal apoptosis inhibitory protein) and survivin (human IAP homologue), have been found to contain a RING finger motif.
The mechanism of action of an IAP protein is complex. While viral IAP homologues block apoptotic cell death, this is not always the case for the cellular homologues. However, several cellular homologues do possess the ability to block apoptosis. For example, two human IAPs, c-IAP1 and c-IAP2, have been identified as components of the TNF (tumor necrosis factor) receptor signalling complex. These c-IAPs interact with the TRAF-N domain of TRAF1 (TNF receptor associated factor 2) and TRAF2 via the BIRs (Rothe et al., Cell, 83:1243-1252, 1995). While the exact function of this interaction is still unknown, recent reports show that c-IAP2 is involved in protecting cells from TNF-induced cell death by activating NF-.kappa.B (Chu et al., PNAS, 94:10057-10062, 1997). Additionally, c-IAP1 and c-IAP2 inhibit cell death by interfering with specific members of the caspase family of cell death proteases, thereby promoting cell survival (Roy et al., EMBO J, 16:6914-6925, 1997).